Descriptor protocol documentation for super bindings is incorrect

[JoshuaArnold]

BPO	10988
Nosy	@rhettinger

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2011-03-22.22:34:40.528>
created_at = <Date 2011-01-23.18:38:03.466>
labels = ['docs']
title = 'Descriptor protocol documentation for super bindings is incorrect'
updated_at = <Date 2011-03-22.22:34:40.526>
user = 'https://bugs.python.org/JoshuaArnold'

bugs.python.org fields:

activity = <Date 2011-03-22.22:34:40.526>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2011-03-22.22:34:40.528>
closer = 'rhettinger'
components = ['Documentation']
creation = <Date 2011-01-23.18:38:03.466>
creator = 'Joshua.Arnold'
dependencies = []
files = []
hgrepos = []
issue_num = 10988
keywords = []
message_count = 4.0
messages = ['126895', '131795', '131796', '131797']
nosy_count = 4.0
nosy_names = ['rhettinger', 'docs@python', 'Joshua.Arnold', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue10988'
versions = ['Python 3.1', 'Python 2.7', 'Python 3.2', 'Python 3.3']

[JoshuaArnold]

In 'Doc/reference/datamodel.rst', the 'Invoking Descriptors' documentation specifies the following behavior for super objects:

[snip]
Super Binding
If ``a`` is an instance of :class:`super`, then the binding ``super(B,
obj).m()`` searches ``obj.__class__.__mro__`` for the base class ``A``
immediately preceding ``B`` and then invokes the descriptor with the call:
``A.__dict__['m'].__get__(obj, A)``.
[snip]

In the above paragrah, the call:

A.__dict__['m'].__get__(obj, A)

is incorrect. In reality, the descriptor is invoked via:

A.__dict__['m'].__get__(obj, obj.__class__)

Loosely speaking, the 'owner' argument is set to the subclass associated with the super object, not the superclass. There is a similar error in the 'Descriptor HowTo Guide' under 'Invoking Descriptors'

(Strictly speaking, the specification is inaccurate on some other points. It assumes obj is not a class and doesn't state that the entire mro preceding 'B' is searched for 'm'. But those may be considered simplifications for the sake of brevity)

I considered reporting this as a bug in the implementation rather than the specification. But I think the implementation's algorithm is the correct one. In particular, it yields the desired behavior for classmethods In any case, the current behavior has been around for a while (it was the fix for issue bpo-535444, dating back to 2.2)

Code demonstrating the current behavior:

class Desc(object):
    def __get__(self, instance, owner):
        return owner

class A(object):
    attr = Desc()

class B(A):
    pass

class C(B):
    pass

instance = C()

assert super(B,instance).attr == C
assert super(B,C).attr == C

#According to the specification, the assertions should be:
#assert super(B,instance).attr == A
#assert super(B,C).attr == A

[python-dev]

New changeset 50cc60852a76 by Raymond Hettinger in branch '2.7':
bpo-10988: fix description of super's descriptor call.
http://hg.python.org/cpython/rev/50cc60852a76

[python-dev]

New changeset 3e3c46a3dce8 by Raymond Hettinger in branch '3.1':
bpo-10988: fix description of super's descriptor call.
http://hg.python.org/cpython/rev/3e3c46a3dce8

New changeset 40698c68a32c by Raymond Hettinger in branch '3.2':
bpo-10988: fix description of super's descriptor call.
http://hg.python.org/cpython/rev/40698c68a32c

New changeset 29c8eb206076 by Raymond Hettinger in branch 'default':
bpo-10988: fix description of super's descriptor call.
http://hg.python.org/cpython/rev/29c8eb206076

[rhettinger]

Fixed.
Thanks for the bug report.